The present invention relates generally to the field of airborne vehicles, and more specifically to a guidance system capable of being used on a small airborne projectile such as a bullet.
The path of a gun-launched projectile is at the mercy of gravity, air currents, muzzle accuracy, barrel wear, sighting accuracy, gun stability, projectile anomalies, charge uniformity, etc. As with a golfer who leans after a shot to encourage the ball to travel one way or the other, one would similarly like to influence the flight path of a bullet to overcome the above disturbances and to deliver the projectile to its intended target. The least expensive weapon for the last several centuries has been a bullet. Although bullets themselves are very inexpensive, they are not always the most cost effective. That is, the real cost of using a weapon includes the cost of all the ammunition plus the cost of delivery necessary to achieve the desired objective. Thus, a more expensive bullet that has a greater accuracy may actually be a less expensive bullet to use.
Most bullets spin about their axis of flight and are thereby spin stabilized. Equipping such a projectile with guidance vanes or other control devices would be useless unless the control devices could be activated only at such times and for an appropriate duration when they could impose the control force in the appropriate direction, and then be retracted when their affect would be inappropriate or counter to the desired flight path correction. Obviously such operation would mean very rapid projection and retraction of the guiding aspects, i.e. a wide bandwidth control system. Traditional control systems are not capable of such rapid deployment. To avoid the need for such a wide bandwidth control system, it is known to de-spin the section of the projectile that houses the control devices. The de-spun section may then be roll stabilized with respect to inertial space. In such a state, the control section moving axially through the air could activate relatively slow moving control devices without subjecting them to the roll of the bullet.
Micro electromechanical systems (MEMS) have been developed based upon a variety of technologies for a variety of applications. An electrostatic actuator using a rolling electrode is described in U.S. Pat. No. 4,266 339 issued to Kalt on May 12, 1981, for application as an electronic window blind.
MEMS actuators have been tested on military aircraft as part of a flight control system for reducing the buffeting forces imposed on the aircraft vertical fin resulting from local flow condition instability. Piezoelectric actuators were used in this test. Although the speed of movement of such actuators is sufficiently high to respond to local flow instabilities, the effectiveness of such piezoelectric actuators is limited because the range of motion of a piezoelectric material is relatively small.
There is a particular need for a guidance control system that is small enough and fast-acting enough to be applied to a projectile as small as a bullet. Accordingly, an airborne vehicle is described herein as having a housing; a plurality of micro electromechanical actuators attached to the housing, each actuator having a flap portion adapted to move between a withdrawn position and an extended position; and actuator circuitry connected to the actuators for selectively moving ones of the flap portions into and out of an air stream passing over the projectile. The airborne vehicle may further include a rotation sensor for producing a first signal corresponding to the rotation of the projectile about an axis of flight; a lateral acceleration sensor for producing a second signal corresponding to acceleration of the projectile in a direction normal to the axis of flight; and control circuitry connected to the rotation sensor and to the lateral acceleration sensor for providing a third signal to the actuators responsive to the first and the second signals. The plurality of actuators may be arranged about the axis of flight, and wherein the third signal is operable to extend selected ones of the flap portions to produce a standing wave of extended flap portions relative to the axis of flight.
A method of controlling the trajectory of an airborne vehicle is describe herein as including the steps of: providing a plurality of micro electromechanical actuators on a projectile, each actuator having a flap portion adapted to move between a withdrawn position and an extended position; determining a desired change in trajectory of the projectile relative to an axis of flight; and actuating a selected portion of the actuators to extend the respective flap portions into and out of an air stream passing over the projectile to achieve the desired change in trajectory. The method may further include the steps of: disposing the actuators on the airborne vehicle about the axis of flight; sensing rotation of the projectile about the axis of flight; and actuating the selected portion of the actuators in a sequence responsive to the rotation of the projectile to form a standing wave of extended flap portions relative to the axis of flight.